Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appl. Phys. Lett 51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, Feb. 1995). Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in International Patent Application No. PCT/US95/15790. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg--Ag--ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg--Ag--ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color. This colored emission could be transmitted through the adjacently stacked transparent, independently addressable, organic layer, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
The PCT/US95/15790 application disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. The PCT/US95/15790 application, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
Such devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material's ability to transport holes, a "hole transporting layer" (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an "electron transporting layer" (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the hole transporting layer, while the cathode injects electrons into the electron transporting layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism. Under this view of the mechanism of operation of typical thin-layer organic devices, the electroluminescent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from each electrode.
The materials that function as the electron transporting layer of the OLED are frequently the same materials that are incorporated into the OLED to produce the electroluminescent emission. Such devices are referred to as having a single heterostructure. Alternatively, the electroluminescent material may be present in a separate emissive layer between the hole transporting layer and the electron transporting layer in what is referred to as a double heterostructure.
A well documented cause of OLED failure is thermally induced deformation of the organic layers (e.g. melting, glass formation, etc.). This failure mode can be seen in the studies that have been carried out with hole transporting materials, K. Naito and A. Miura, J. Phys. Chem. (1993), 97, 6240-6248; S. Tokito, H. Tanaka, A. Okada and Y. Taga. Appl. Phys. Lett. (1996), 69, (7), 878-880; Y. Shirota, T Kobata and N. Noma, Chem. Lett. (1989), 1145-1148; T. Noda, I. Imae, N. Noma and Y. Shirota, Adv. Mater. (1997), 9, No. 3; E. Han, L. Do, M. Fujihira, H. Inada and Y. Shirota, J. Appl. Phys. (1996), 80, (6) 3297-701; T. Noda, H. Ogawa, N. Noma and Y. Shirota, Appl. Phys. Lett. (1997), 70, (6), 699-701; S. Van Slyke, C. Chen and C. Tang, Appl. Phys. Lett. (1996), 69, 15, 2160-2162; and U.S. Pat. No. 5,061,569. The most common hole transporter used in the HTL of OLEDs is a biphenyl bridged diamine, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1-biphenyl-4,4'-diamine (TPD) having the chemical structure: ##STR2##
This material has a melting point of 167.degree. C. and a glass transition temperature of 65.degree. C. If a device prepared with TPD is heated above 65.degree. C., catastrophic irreversible failure results. In order to increase the glass transition temperature of the HTL, several groups have explored different modifications to the basic structure of TPD, Naito et al.; Tokito et al.; Shirota et al.; Noda et al. (Adv. Mater.); Han et al.; Noda et al.(Appl. Phys. Lett.); Van Slyke et al.; and U.S. Pat. No. 5,061,569. While these studies have led to materials with T.sub.g values as high as 150.degree. C., they have not led to an understanding of why certain structural modifications increase T.sub.g, while other modifications may not affect T.sub.g at all or may even lower T.sub.g. Still other modifications may produce a material not having a glass transition temperature at all or a material not having the combination of properties that is suitable for use in an HTL. For example, replacing the amine groups of TPD with carbazole groups to produce 4,4'-di(N-carbazolo)diphenyl (CBP), having the chemical structure: ##STR3##
increases the melting point to 285.degree. C. However, the material shows no glass transition. Materials that are present as a glass are desirable for use in the HTL of an OLED, rather than as a crystalline or polycrystalline material, since glasses are capable of providing higher transparency as well as producing superior overall charge carrier characteristics as compared with the polycrystalline materials that are typically produced when thin films of the crystalline form of the materials are prepared.
U.S. Pat. No. 5,061,569 discloses hole transporting materials comprised of at least two tertiary amine moieties and further including an aromatic moiety containing at least two fused aromatic rings attached to the tertiary amine nitrogen atoms. Out of the large number of compounds encompassed by the broadly disclosed class of compounds recited, U.S. Pat. No. 5,061,569 fails to disclose how to select those compounds which have a high glass transition temperature. For example, the naphthyl derivatives do make stable glasses, however, the .alpha.-form (containing the 1-substituted naphthyl group), a compound (NPD) having the chemical structure: ##STR4##
was reported to have a T.sub.g (105.degree. C.) which is substantially higher than TPD, while the .beta.-form (containing the 2-substituted naphthyl group), a compound (referred to herein as the .beta.-derivative of NPD) having the structure: ##STR5##
has been generally understood to have a T.sub.g which is substantially lower than .alpha.-derivative. Apparently because of this purportedly low and anomalous difference between T.sub.g of the .alpha.- .beta.-derivatives, there are no known reports of using the .beta.-derivative as the hole transporting material of an OLED.
It would be desirable if OLED's could be fabricated from glassy charge carrier materials having improved temperature stability, while still providing luminescent characteristics comparable to prior art compounds. In addition, it would be useful to have a method for selecting and preparing such glassy charge carrier materials having improved temperature stability, as characterized, in particular, by glassy charge carrier materials having a high glass transition temperature.